Stability of Metal Matrix Composite Pads During High-Speed Braking

Metal matrix composites are now commonly used as braking pads for the train running over 250 km/h by virtue of a number of desirable properties. To develop a fundamental understanding of the stability of metallic composites at high-speed braking, four typical composite materials, with different Cu and Fe contents, were subjected to a series of high-speed emergency braking at a simulative running speed of 380 km/h and a braking inertia of 27 kg/m−2 and a normal pressure of 1.27 MPa in this paper. The results showed that the sample with higher Cu content displayed a fade COF and deteriorated wear, but the one with higher Fe content could maintain a stable COF and low wear rate. The tribological behaviour is associated with the relative rate of generation and consumption of the tribo-oxide film. For the sample with higher Cu content, the generation rate of tribo-oxide film was less than the consumption rate, and the COF fading and wear deterioration with the increasing braking times were attributed to the reduction in resistance to deform or to shear the asperities, which was thought to be caused by the degradation of near-surface layer due to the removal of protective tribo-oxide film. In contrast, for the sample with higher Fe content, the generation rate was approximately equal to the consumption rate, and a well-established tribo-oxide film on the surface was responsible for the stable friction level and low wear rates.